1. Field of the Invention
This invention relates generally to field flow fractionation technology. More particularly this invention relates to a method of and apparatus for electric field flow fractionation wherein the fractionation flow channel is electrically insulated from the applied electrical field.
2. Related Art
Field flow fractionation (FFF) was first described in the patent literature in U.S. Pat. No. 3,449,938 (Giddings). Fractionation of components in a mixture was achieved by applying a temperature gradient between the top and bottom plates of a fractionation flow channel. Since then, separation has been achieved by the use of various types of force fields. A general method for separation by FFF is: A ribbon shaped flow channel is created by sandwiching a shaped gasket between two parallel plates (channel walls). The channel is typically long in the axial dimension, i.e., for analytical purposes, about 10-100 cm long in the direction of fluid flow. A typical channel has a width of 1-2 cm and a height of 25-200 xcexcm. The fractionation field is imposed perpendicular to the length and width, and parallel to the channel height. Due to capillary effects, a parabolic flow profile develops between the top and bottom plates. A sample is injected into the carrier stream prior to entering the channel, and the sample components are monitored downstream of the channel exit port.
With conditions of laminar flow, the fluid flow velocity within the channel is a function of distance from the channel walls. The fluid flow velocity is at a maximum at a position midway between the top and bottom plates, and is at a minimum at the channel walls. When a fractionation field is applied perpendicular to the direction of laminar flow, any particle/molecule that interacts with the force will be forced to one or the other wall of the channel. However, particle/molecule accumulation at a wall cannot continue indefinitely, as particulate/molecular mass diffusion acts to counter the buildup of concentration at the wall. The two competing processes come to equilibrium, creating a Gaussian concentration distribution at a characteristic distance from the wall. This characteristic distance depends on the type of particle/molecule and its interaction strength with the field, and the particles/molecules diffusion rate in the carrier. A particulate/molecular distribution centered close to the wall will be in a slower moving laminae than one centered midway between the walls. The distribution centered midway between the walls will be moving faster through the channel and thus, it will exit the channel prior to the distribution centered near the wall.
Early demonstrations of an electric field applied to an FFF flow channel used a semipermeable membrane for the channel walls, with the electrodes positioned externally A later EFFF channel design used graphite plate electrodes, (U.S. Pat. No. 5,240,618 Caldwell et al.) The carrier solution used in this latter case was either deionized water or an aqueous solution containing a red-ox couple such as quinone/hydroquinone. In both of these examples there was an electrical current flow across the channel.
The present invention is an apparatus and a process for separation and resolution of particles suspended in, or molecules dissolved in, a sample mixture using electrical field flow fractionation (EFFF). Fractionation of individual components in a mixture/solution is obtained by the interaction of particles/molecules with an electric field applied perpendicular to the flow direction, and externally to the fractionation flow channel. A parabolic flow profile is established between two conducting plate electrodes. The plate electrodes are electrically isolated from the sample and carrier with a thin, non-permeable, insulating coating on the inside surfaces of the electrodes (channel walls). This coating forms a barrier between the solution phase and the electric circuit used to generate the applied electric field.
The flow channel is formed by sandwiching a shaped insulating gasket between the two zparallel plate electrodes. The side walls of the channel are defined then by the inside walls of the shaped gasket. The top and bottom walls are formed by the two, coated, parallel plate electrodes. The channel has an inlet port at one end, and an outlet port at the opposite end. A carrier fluid comprising either water or an organic solvent is pumped in the channel through the inlet port, and it exits out the outlet port. A sample is mixed with the carrier liquid prior to entering the channel and the sample is monitored for separation of the particles/molecules downstream of the exit port.